In an HTGR power plant a cooling gas (helium) is circulated through the reactor whenever the reactor operates. In an indirect cycle HTGR power plant, hot reactor cooling gas flows from the reactor to the primary sides of a plurality of steam generators which derive heat from the gas, and supply superheated and reheated steam to a turbine-generator. For desirable operation and protection of the steam generators it is necessary to maintain a minimum steam flow through the superheater and reheater sections of each steam generator. Typically, the total minimum steam flow through the steam generators is sufficient to generate 25% of maximum plant power. Therefore, bypass lines are connected across the various turbine elements to permit the total minimum steam flow through the steam generators at times when the steam flow through the turbine elements is less than such minimum.
A helium circulator is associated with each steam generator to circulate a cooling gas through the reactor and the respective steam generator. Such a circulator may be rotated by an auxiliary steam turbine. When auxiliary steam turbines are so utilized, an auxiliary turbine provided for each helium circulator uses a portion of the steam flowing to the inlet of the reheater section of the associated steam generator. The outlets of the reheater sections are commonly connected to a hot reheat header. Reheated steam may flow from the hot reheat header in three paths. A first path comprises an intercept valve and a lower pressure turbine element, a second path comprises a condenser bypass line and bypass valve means, and a third path comprises an alternate bypass line and valve means therein connected. Regulation of the hot reheat header steam pressure improves control of the shaft speed of the auxiliary steam turbines and thus permits improved control of the flow rates of reactor coolant gas.
For purposes of loading the turbine-generator, the steam flow through the high pressure element is controlled by positioning an associated governor valve, and the steam flow through the intermediate low pressure element is controlled by positioning an associated intercept valve. Simultaneously the bypass valves connected in the bypass lines associated with the intermediate-low pressure element must be positioned to maintain the desired minimum steam flow through the reheater sections of the steam generators at times when the steam flow through the intermediate low pressure element is less than such minimum.
In a prior art system for loading a turbine-generator in a HTGR power plant, an intercept and a bypass flow valve are interlocked, whereby one valve opens as the other valve closes. Ideally, the two valves are designed to present a constant total resistance to steam flow at any interlocked position so that the steam pressure in the hot reheat header remains substantially constant under conditions of constant steam flow from the reheaters. In practice, the steam flow from the reheaters may vary, or the total resistance of the intercept and bypass valves may vary, causing variation of the hot reheat header steam pressure. In event of a turbine trip, a pressure relief valve may open to relieve excessive hot reheat header steam pressure and close thereafter at a predetermined pressure level; however, such on-off control typically permits fluctuation of the post-trip hot reheat header steam pressure, with possible deterioration of the accuracy of control of the reactor coolant gas flow rates.
In a proposed system for loading a turbine-generator in an HTGR power plant the steam flow through the high pressure element is varied to reduce a difference between a detected pressure of steam in the impulse chamber of the high pressure element and a desired value of such pressure that is in accordance with the desired power output of the turbine-generator, while the steam flow through the intermediate-low pressure element is varied to reduce a difference between a detected pressure of steam in the first stage of the intermediate pressure element and a desired value of such pressure that corresponds to the desired turbine-generator power output. While such a system satisfactorily controls the power output of the intermediate-low pressure element, the power output of the high pressure element is controlled inaccurately at low load levels (when the intercept valve is not fully opened) as the relationship between the detected impulse chamber steam pressure and the power output of the high pressure element at such load levels is nonlinear and variable with hot reheat header steam pressure. In this control system the position of the bypass valve means is varied in accordance with a signal which comprises a first component proportional to the pressure difference and a second component proportional to the time intergral of the pressure difference. Such a system operates satisfactorily in an HTGR power plant which includes a single turbine-generator. However, such a control system has certain limitations when two such control systems operate in concert, as in an HTGR power plant which includes two bypass systems and two turbine-generators, for example. When two such systems are so utilized two integrators simultaneously integrate a pressure difference signal, and the integrator output signals, which ideally are equal, frequently diverge in practice causing undesirable imbalance between the steam flows through the bypass lines.
It is desirable to provide a turbine-generator load control system which may be used in a dual turbine HTGR power plant without causing unwanted imbalances between the turbine bypass line steam flows, and which also may be used in a single turbine plant. It is also desirable to provide such a load control system that accurately controls the power output of the high pressure element at low load levels to provide such a load control system which simultaneously positions a condenser bypass valve means and an alternate bypass valve means in both a single and a dual turbine HTGR power plant. It is further desirable to operate the alternate bypass valve means to regulate the condenser bypass line steam flow at a limit which varies according to plant operating conditions. It is advantageous to provide a load control system which governs turbine bypass steam flow in response to a difference between detected and desired values of such pressure, as such a system controls the steam pressure despite variation of the steam flow through the reheaters. It is advantageous to provide a load control system which does not utilize interlocked control valves.